1. Field of the Invention
This invention relates to mass data storage and retrieval and, in particular, mass data storage in a Redundant Array of Inexpensive Tape drives (RAIT) wherein a master tape drive controls a plurality of slave tape drives.
2. Related Art
Among the types of mass data storage devices are arrays of tape drives. Typically such tape arrays inherently suffer from synchronization problems. The lack of synchronization causes severe degradation of data throughput for such arrays. In addition to extending backup times, spurious, array-induced tape repositioning decreases the capacity of the array. The overall result is a drastically lower performance than expected. While disk drive arrays enjoy proportional increase in throughput and capacity in respect to the number of drives present in the array, tape RAIT systems often perform at a fraction of their theoretical potential. Tape repositioning can be caused by a number of problems, including the loss of streaming operation due to different data compression ratios experienced by individual units within an array, different media characteristics in each array unit (i.e. different incidence of error rates and error severity), and different head contamination levels and mechanical alignment/calibration for each array unit (i.e. different incidence of error rates again).
Media access errors often result in the so called Read After Write (RAW) error recovery operation, where a device re-writes the same information repeatedly until a successful write is accomplished. This extends both the time of the write operation and the amount of media used to store the given amount of information. In an array, a drive conducting a Read After Write operation will likely induce repositioning behavior in the remaining drives within the array. Media access errors during read operations may cause the drive to attempt to re-read the information, which is accomplished by repetitive reposition/read cycles. The detrimental effect on the array is similar to the RAW side-effects. Reading media which contain RAW frames disproportionately extends the read operation for the specific unit which encounters such frames, which again causes the synchronization among tape drives in an array to be lost.
Host induced repositioning due to falling source data rates triggers repositioning, which is likely unsynchronized among drives (they start and complete repositioning cycles at different times), thus causing artificial throughput degradation on top of already lower rates.
Additionally, a number of other factors compound the synchronization problems. Repositioning action by a tape drive has a "ripple effect" on the array as a whole: a single drive blocks the sustained data transfer to the array and causes remaining drives to reposition as well. Since the effect is "cascaded" over time, the drive that finishes repositioning may yet enter another repositioning cycle while waiting for the other drives to finish their repositioning. In the worst cases, the array may not recover from this behavior for a long time, if at all.
Also, repositioning characteristics are unit-specific; this means that individual drives will likely require different amounts of time to perform repositioning cycles. Repositioning cycles also differ in duration within the same unit. Consequently, repositioning is largely unpredictable, and even though average times can be assessed, average performance levels carry no practical implications for the dynamic requirements of high-speed arrays.
The "cascading" effect described above can further be worsened by individual units unable to catch up with the remaining drives in the array. In the worst case, the array may not stream altogether despite the fact that the host is supplying data at the rate sufficient to support streaming operation.
As seen from these observations, the recovery from multi-drive repositioning phenomena is difficult, time-consuming, inefficient, and not always certain. The current, and not very successful, attempts at making tape arrays perform acceptably are: (1) increasing on-board buffer capacities in tape drives; (2) turning data compression off, (3) placing artificial caps on maximum compression ratios (which is done by writing blank areas on tape, thus severely limiting the capacity); (4) aborting backup sessions when media does not perform at the highest levels, and restarting the sessions with new media. As it can easily be surmised, these solutions are far from optimal, and are at best jury-rig attempts at making tape arrays marketable at all. The general outcome is that tape arrays are uncommon in the marketplace and the customer satisfaction levels are disappointing.